This invention relates in general to a breathing mask construction and in particular to a new and useful means for transmitting speech from the interior of the mask to the exterior.
Several ways are known to effect speech transmission from protective breathing masks A known typical system is described in German OS No. 30 13 939 disclosing a microphone loudspeaker optionally usable for a breathing mask or a protective helmet. The microphone loudspeaker comprising an electroacoustic, preferably dynamic, transducer, is accommodated in a cup shaped housing which is detachably secured by its cylindrical edge to the rim of the exhaling valve housing of a mask, or to the chin piece of a helmet. The sole advantage of such an arrangement is that one and the same transducer system can be used both as a microphone and as a loudspeaker. A microphone loudspeaker, however, has always the drawback of a reduced transmission quality, for acoustic reasons. Further, the microphone receives only speech passing through the mask, which is thereby made considerably unintelligible. On the other hand, sound coming from the outside of the mask thus predominantly noise, such as from the exhaling valve, is received without attenuation or distortion. To obtain a satisfactorily intelligible sound, the antechamber of the exhaling valve must be designed as a Helmholtz resonator, with a resonance frequency at about 2,400 Hz, and another Helmholtz resonator must be provided having a resonance frequency of 3,000 Hz, to ensure a transmission at least up to 3,000 Hz.
The poor reproducing quality of microphone loudspeakers is due to the fact that the vibratory conditions of the diaphragm at the reception are different from those at the reproduction, and electrical measures are needed to balance the frequency response of the two transducers. Further, a diaphragm diameter of 3 to 4 cm, usual in such microphone loudspeakers, is not sufficiently responsive to low frequencies. The distortion factor of such small loudspeaker systems also is correspondingly high. This must be taken into account while appreciating the use of such a system under emergency conditions where intelligibility of speech is imperative and misunderstanding may be fatal.
Mentioned German OS No. 30 13 939 describes various devices making possible understanding between two persons. Mentioned are portable radio transmitters-receivers, compact transceivers, throat microphones, bone-conduction microphones, and microphones secured to the exhaling valve of the mask, which all have some disadvantages. For example, to handle a transmitter-receiver, the user must have one hand free, which may considerably hinder his activities. Or the arm carrying the microphone of a transceiver must be swung to the mouth region upon putting on a mask, since only there, the speech poorly intelligible through the mask can be received. The other mentioned systems have mostly insufficient acoustics and are inconvenient to carry, or have their speech signals to strongly affected by the ambient noise, particularly the operation of the valve of the mask.
In German OS No. 31 37 113, a helmet mask arrangement equipped with a contact microphone is mentioned, however, what is meant is a microphone such as described in German OS No. 30 13 939, directly picking up vibrations which occur on the head during the act of talking. From Austrian patent No. 342,129, a gas or smoke mask is known in which a microphone provided close to the mouth region within the facepiece of a gas mask is exposed to the sound waves produced by the speech. Since no speaking diaphragm is provided in this mask, the built-in microphone must be used also for close range communication. This may strongly reduce the possibility of understanding, since the microphone may come into direct contact with the user's cheek, quite aside from the acoustically unsatisfactory position of the microphone laterally of the mouth which strongly dampens the high frequencies substantially contributing to the intelligibility of spoken words.
German OS No. 31 27 677 discloses a speaking device for mask users providing a transmission arrangement which is secured at least to the outside of the mask and capable of producing an output signal corresponding to the voice of the mask user. The output signal may then be supplied to a loudspeaker carried on the user's body, to produce sound signals which are audible to persons present in the vicinity of the mask user. Evidently, this prior art device does not comprise a speaking diaphragm for close-range communication, it rather requires a complicated electroacoustic arrangement for this purpose. Aside therefrom, in the prior art device, the sound must penetrate through the material of the mask, which does by no means contribute to a distinct comprehension of what was spoken behind the mask. Also, the obtained result is certainly not adequate to the considerable costs of the construction.
German AS No. 17 08 045 describes a mask attachment comprising at least one exhaling valve and a speaking diaphragm. The diaphragm is intended for close-range communication only, and an inner mask is provided preventing the outside air from passing directly to the window, while the sound of speech is conducted to a speaking diaphragm and passes through an antechamber to the outside. With this arrangement, the sound cannot be transmitted over a longer distance.